In elevator installations steel cables were conventionally used as support means for supporting and/or driving an elevator car. According to a development of such steel cables belt-like support means comprising tensile carriers and a casing arranged around the tensile carriers are also used. However, such belt-like support means cannot be monitored in conventional manner, because the tensile carriers, which determine a breakage load of the support means, are not visible through the casing.
For the monitoring of such tensile carriers in belt-like support means a test current can be applied to the tensile carriers. A current flow or an amperage, a voltage, an electrical resistance or an electrical conductivity is measured in the thus-formed current circuit or in several thus-formed current circuits. A conclusion about the intact state or degree of wear of the support means can be made on the basis of a variable measured in that way. If, in particular, the diameter of a tensile carrier reduces due to breakage of individual wires or due to metallic abrasion the electrical resistance of this tensile carrier increases.
Published specification DE 3934654 A1 discloses, for example, a serial connection of all individual tensile carriers and an ammeter, wherein it is checked whether an electric current flows through the tensile carriers connected in series. It can thereby be established whether one of the tensile carriers is interrupted at some point.
Patent U.S. Pat. No. 7,123,030 B2 discloses calculation of an electrical resistance by measurement of an instantaneous voltage by means of a Kelvin bridge and comparison of the thus-determined voltage value with an input reference value.
Publication WO 2005/094250 A2 discloses temperature-dependent measurement of electrical resistance or electrical conductivity, in which different environmental and thus also presumed support-means temperatures are taken into account, which can be very different particularly in high elevator shafts.
WO 2003/059798 A2 discloses a belt-like support means for elevator installations. In that case, a conclusion about the loading of the support means and thus about loading of the car can be made by determining an electrical resistance of the tensile carriers of the support means. In addition, breakages of tensile carriers can be ascertained through the resistance measurement if an infinitely high resistance is ascertained. Use can be made of, for example, a Wheatstone measuring bridge for determination of the resistance.
However, it is disadvantageous with all these known forms of monitoring of support means that environmental influences which influence the electrical resistance of a tensile carrier are taken into consideration only to an insufficient degree. Thus, apart from temperature also other factors have an influence on the conductivity of the tensile carriers, such as, for example, magnetic fields or humidity. The known forms of monitoring therefore do not provide reliable statements about the state of the support means.